The invention described herein was made in the course of, or under, a contract with the U.S. ATOMIC ENERGY COMMISSION. It relates to a barrier for inhibiting hydrogen diffusion from a nuclear fuel element containing a metal hydride moderator and operating at temperatures at which a substantial hydrogen dissociation pressure exists. More specifically, it relates to a hydrogen barrier consisting of a liquid metal such as liquid lead.
Zirconium hydride, which has many desirable features as a moderating material for use in SNAP space reactor systems, has the serious disadvantage of producing substantial hydrogen dissociation pressure at the cooling temperatures (600.degree. to 800.degree. C) normally used with SNAP reactors. Standard cladding materials such as iron or nickel alloys are relatively transparent to hydrogen at these temperatures so that a barrier is required to prevent a serious loss of hydrogen over a period of time. Such a loss of hydrogen can significantly affect the neutronics within the reactor system.
In searching for appropriate barrier materials, it is evident that one must select materials in which the permeability of hydrogen is low, preferably as near zero as possible. This represents a formidable problem especially at higher temperatures. In addition, containment of hydride moderating material requires that the thermal conductivity of a barrier material be as high as possible since heat is generated within the moderator by neutrons and gamma rays. If the moderator also contains dispersed fissile fuel, as it does in the SNAP-8 reactors, heat is also produced by fissions.
It is known in the art that, insofar as SNAP reactor systems are concerned, a thin ceramic layer applied to the inner wall of the cladding will effectively reduce hydrogen leakage to tolerable rates. However, these ceramic layers have certain serious disadvantages in that they are difficult to apply, they are stable only on the inside surfaces of tubes, and they are subject to cracking damage from hydride swelling, shock, radiation, or if the fuel element bows excessively.